Gamma-Ray Bursts • Energy problem and beaming * • Mergers versus collapsars • GRB host galaxies and locations within galaxy • Supernova connection • Fireball model * • Swift • Afterglows of short bursts Energy Problem • GRB 990123 required a total energy, if isotropic, of 3.41054 erg = 1.9 M c2. • GRB energy source is almost certainly gravitational – need few M collapsed into region not more than 100 km across. • Energy density U = T4/c T ~ 1012 K kT ~ 100 MeV This is high enough to produce e+e- pairs. Fireball • Consider pure energy confined within a sphere, describe with E, R, T (Goodman 1986) • Radius of sphere R = (3E/11T4)1/3 • Optical depth from center to edge 1/ 3 T 15 E kT 4 5/3 kT T R 4 10 51 10 erg 1 MeV • Edge of sphere (photosphere) will expand at a speed close to c as long as kT > mec2 • If baryons are added, most energy goes into accelerating baryons to ~ E/Mc2 Fireball • Optical depth from center to edge for burst which varies over time scale t with a sepctrum such that a fraction fp of the photon pairs can pair produce. 2 E T 10 f p 50 11 10 erg 10 ms • Very high optical depth is inconsistent with non- thermal spectrum at high energies Relativistic outflow • In a relativistic outflow, the observed photon energy is a factor (= Lorentz factor of bulk motion) higher than the photon energy in the rest frame. For a spectrum with an energy index this reduces the number of photon pairs above the electron-positron threshold by –2 • Also the size of the emitting area can be larger by a factor 2 ct R v c 1 ct vt 2 t 2 2 2c 2 • Need ~ 100 to solve the problem. Evidence for Jet Afterglow of GRB 990123 shows a break Beaming Because of relativistic motion, radiation is beamed with an opening angle ~ 1/ Therefore, observer can see only a limited piece of an expanding shell Observer 1/ At Early time: 1 = jet angle 1 Area visible to an observer = (R/)2 R At Late time: 1 1 Area visible to an observer = (R)2 R Monochromatic Jet Break Jet Breaks • Jet opening angle is related to time at which break in light curve occurs • Beaming fraction is determined by jet opening angle = 1 – cos 2/2 • Energy required is reduced by a factor 2/2 Jet Energy Frail et al. 2001 Burst Models • Collapsing WDs • Stars Accreting on AGN • White Holes • Cosmic Strings • Black Hole Accretion Disks I) Binary Mergers II) Collapsing Stars Mergers Binaries must evolve before merger and binaries have non-zero speeds due to kicks in compact object formation. Thus, GRBs can occur in outskirts of or even far from host galaxy. Massive Star Collapse Beamed Explosion, accompanying supernova-like explosion, GRBs should be associated with young, massive stars. Host Galaxies Holland 2001 Hosts are similar to star-forming galaxies at similar redshifts. High star formation rates. Location of GRB within Host Location of Distribution GRB within Follows Host Stellar Distribution The environments of GRBs show higher gas densities, higher metallicities, and higher dust content than random locations in host galaxies. Suggests that GRBs occur in star forming regions. GRB Locations • GRB hosts are star-forming galaxies • GRBs trace the stellar distribution (in distance from galaxy center) • GRBs occur in dense environments (probably star forming regions) • Suggests collapsar model over merger model Supernova connection SN 1998bw was found in the 8’ error circle of GRB 980425 in observations made 2.5 days after the burst. A slowly decaying X-ray source was subsequently found in the same galaxy (z = 0.0085) and identified with the GRB. However, the GRB was very underluminous and the SN was very usual with parculiar line emission (no H, no He, no Si at 615 nm. Radio emission a few days after GRB indicated relativistic outflow with energy ~ 31050 erg. Thought to be oddball GRB and SN. GRB030329 and SN 2003dh Clear spectroscopic signature of a SN, broad emission lines, found after decay of afterglow of GRB030329. “Smoking gun” linking GRBs and SNe. SN 2003dh versus SN 1998bw SN Bumps GRB - Supernova Only a tiny fraction of SN are observed to be GRBs GRB 060218 = SN 2006aj Fireball Model Initial event accelerates baryons in bulk Later on, internal shocks re-accelerate particles produce GRB Even later, external shocks produce afterglow GRB 990123 990123 reached 9th magnitude for a few moments! First optical GRB afterglow detected simultaneously Internal-External Shock Model External Shocks Internal Shocks ISM Central Engine R 1014 cm R 1016 cm GRB Afterglow ３ Burst (as Jet) Properties 3. Baryonic mass content of the jet ~ 2x10-7 6x10-6 Mo Baryon mass is ~ 10-5 M Jet opening angle means that we observe only one of each 1000 GRBs in the Universe, most are pointed away. The means that GRB rate is about 1% of SN rate. Swift BAT – CZT detector with 5200 cm2 area sensitive in 15-150 keV band. Coded aperture imaging of 1.4 steradian field with 4 arcmin resolution suing 32768 pixels. After detecting a burst, Swift autonomously repoints bringing the burst into view of the XRT and UVOT, often within 90 seconds. XRT – focusing X-ray telescope in 0.5-6 keV band, 2.5 arcsecond source location accuracy. UVOT – focusing UV/optical telescope. Swift Results • Launched in 2004. • Detects about 100 bursts/year • More afterglow detections than all previous satellites combined • GRB with redshift of z = 6.29 • Average redshift = 2.7 compared to pre-Swift <z> = 1.2 • • Expect 40 GRB with z > 5 and 4 with z > 8. Afterglow of short GRB GRB 050509b associated with elliptical galaxy. HETE-II GRB 050724 also associated with elliptical.
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